This invention relates to imaging methods using a novel group of photochromic compounds. More particularly, this invention relates to a novel group of photochromic compounds characterized by their ability to change and retain their color upon exposure to visible and ultraviolet light. Still more particularly, this invention relates to various novel compositions of matter comprising various materials having dispersed throughout the body thereof at least one of said photochromic compounds.
Photochromic compounds are well-known in the art, as is the usage thereof for such applications as temporary data storage devices, reflectants for incident high-intensity radiation, optical filters and the like. Generally, the prior art photochromic compounds become colored or change color upon exposure to ultraviolet light and automatically revert to their original color when they are removed from the ultraviolet light or stored in the dark. Various other photochromic compounds, however, change color only when exposed to a high degree of irradiation, such as 10-25 cal/cm.sup.2 /sec or more, and as such, sunlight (0.2 cal/cm.sup.2 /sec) will not affect them.
It is usually the reversible nature of the photochromic process which has made it attractive, particularly in the case of memory devices. Ironically, photochromic compounds in spite of many obvious advantages have not found complete acceptance primarily for two reasons, the second of which is surely a consequence of the first such as follows.
A. Activation or coloration of photochromic compounds usually requires the use of light in the ultraviolet region of the electromagnetic spectrum;
B. True reversibility has not been realized with known photochromic compounds. Cyclic fatigue manifests itself as a cessation of photochromic activity after a number of write-erase cycles, short of that required for effective and economical use of these materials.
The first of these problems, that of ultraviolet exposure required for activation, has the obvious technical disadvantage of requiring ultraviolet transmitting components in optical systems used for an imaging process built around photochromics. The cost of such components, were they even available or feasible, would likely more than offset any potential benefit derived from the photochromic material. Thus, the desirability of using photochromic compounds which respond to visible light becomes apparent. Also, the failure of known photochromic compounds to retain their color for more than a few minutes after being subjected to ultraviolet light seriously detracts from their applicability for many commericial and industrial uses. Of prime importance is the need for compounds of this type which can be utilized for permanent or long-term information storage.